ASHRAE Journal - November 2019 - 46

ASHRAE - CELEBRATING 125 YEARS

90 fpm & Laminar Flow

One of the most enduring questions concerning cleanroom standards is "why 90 fpm?" FED-STD-209A, B,
which were in effect for over 20 years, had speciﬁed 90
fpm ±20 fpm (0.5 m/s ±0.1 m/s) in the facility design
guidance. Numerous anecdotes and second- and thirdhand stories speak about that the origins of the velocity
used in Willis Whitﬁeld's laminar ﬂow system. Some had
speculated this was the velocity theoretically calculated
to remove a particle dropped in front of the supply ﬁlter
in the ﬁrst laminar ﬂow room at Sandia Corporation.
Another opinion was that the only air supply fan available to Willis Whitﬁeld produced this air velocity. Another
said that the 90 fpm (0.5 m/s) was the minimum velocity
needed to overcome buoyancy effects of a hot surface in
the workspace.5
90 fpm (0.5 m/s) is almost equal to 1 mph (0.4 m/s), and
velocities greater than 100 fpm (0.5 m/s) can produce a
sensation of draft for some people. Whitﬁeld had said
"The real value of ﬁltered laminar airﬂow is the high
degree of cleanliness that it maintains at very low velocities, well below personnel discomfort levels. The 100 lineal fpm (0.5 m/s) air velocity utilized in these rooms was
well below the rate of 150 fpm to 200 fpm (0.8 m/s to 1.0
m/s), which is generally considered to be the threshold of
personnel discomfort." 13 Others have quoted Whitﬁeld,

and Colebrook in 1946, and Blowers
and Crew in the 1950s, Dr. Charnley
and air-conditioning engineer
James Howarth built one of the ﬁrst
unidirectional airﬂow rooms using
sterile air supply and displacement
ventilation.11,4

Beginnings of Modern Cleanrooms

The watershed event in the history
of the cleanroom was the invention
of the ﬁrst "laminar ﬂow" or true
unidirectional concept of ventilation in 1960 - 1961 by physicist Willis
Whitﬁeld, Ph.D., at the Sandia
Laboratories in Albuquerque, N.M.,
while working with the U.S. Atomic
Energy Commission (AEC).5 In
1959 Whitﬁeld and his team were

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who said that the fans used in his cleanroom could produce between 50 fpm and 200 fpm (0.3 m/s to 1.0 m/s).
50 fpm (0.3 m/s) could not remove particles fast enough
if more than one person was in the room, and above 100
fpm (0.5 m/s) the noise from the fans became annoying.5
This seems more in alignment with Whitﬁeld's explanation in 1963. The most logical answer is a combination
of items, worker comfort (sound and draft) and recovery
rate.
In addition to the origins of 90 fpm (0.5 m/s) and
equally controversial was Whitﬁeld's decision to use the
term laminar ﬂow when describing his unidirectional
airﬂow in the ultra-cleanroom. It was known that from a
purely scientiﬁc basis, airﬂow in this room was not laminar. During an interview in 2005, Willis responded to a
question from Sandia Lab News: "Lab News then asked
just what was 'laminar' about the so-called 'laminar ﬂow
cleanroom' - the usual term used to describe his group's
invention. 'Nothing,' said Willis, who described the word
as a preexisting marketing term and a catchy name. 'The
air is just unidirectional.' " 28 Mr. Whitﬁeld has also said
he very carefully puts 'laminar ﬂow' in quotation marks
when outlining the innovation and refers instead to 'unidirectional air ﬂow.' Whitﬁeld traces the application of
the name to his innovation to the meetings of the group
that devised Standard 209. 21

investigating why cleanrooms could
not stay clean. The problem was that
while previous cleanrooms could
achieve a desired level of cleanliness, they did not remain clean
without continuous cleaning by personnel. The problem was still that
some of the best cleanrooms and
clean hoods would average no better
than approximately 100,000 particles of 0.5 micron and larger per
cubic foot. The focus was on keeping
contaminants out, not on removing
any generated by the work or personnel inside the room.12
New products being manufactured required continuous cleaning,
and the generation of particles was
exceeding the ability of the those

N O V E M B E R 2 0 19

cleaning the space. The problem
was particles generated within the
space stayed in the space unless they
were removed by the cleaning staff.
Previous work was focused on cleaning people, isolating the contamination from people with improved
clothing, continuous cleaning of
work surfaces, disinfecting surfaces
and materials and providing clean
ﬁltered air to the space. Whitﬁeld's
team needed a method to keep providing clean air and to remove the
particles generated within the space.
Whitﬁeld and team's approach to
the problem was to create a "radical
design"-by comparison with a conventional cleanroom. Conventional
cleanroom problems were grouped

ASHRAE Journal - November 2019

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